Effect of Post-Veraison Canopy Reduction and Deficit Irrigation on ‘Merlot’ grape Composition

In several winegrowing regions of the world, climate change has determined an acceleration of the ripening process, resulting in increased sugar concentration at harvest and in wines characterized by high alcohol levels, low acidity and reduced varietal aroma. The limitation of photosynthetic carbon availability by drastically reducing canopy size, has been proposed as a strategy to reduce berry sugar at harvest. However, in some cases, this strategy has negatively impacted other fruit composition parameters (e.g. anthocyanins). Our research investigated the effect of canopy manipulation on sugars and phenolic accumulation in the berries of grapevines subjected to different irrigation regimes. The trial was conducted at the experimental station of the University of Udine (Udine, Italy) on 28-years-old ‘Merlot’ grapevines grafted on SO4 in 2011 and 2012. We tested the effect of a severe post-veraison canopy reduction (-50% of total leaf area) in combination with deficit irrigation to reduce sugar concentration and enhance phenolic concentration in the berries at harvest. Four treatments were tested: 1) Canopy reduction under deficit irrigation (CR-WD), 2) Canopy reduction under full irrigation (CR-I), 3) No canopy reduction under deficit irrigation (C-WD), and 4) No canopy reduction under full irrigation (C-I). In 2011, CR-WD treatment resulted in berries with lower sugar concentration and higher anthocyanin concentration than the other treatments. Canopy reduction was also efficient in reducing sugar without altering anthocyanin concentration under well-watered conditions. In 2012, however, canopy reduction did not reduce sugar concentration at harvest. Our data suggests that modifying the source-sink balance via canopy reduction is a viable practice for modulating sugar accumulation only when the leaf area/crop weight ratio is a limiting factor. Seasonal weather variability may overcome the targeted ratio by compensating fruit size and/or yield and canopy reduction should be managed accordingly.